[unreadable] The spliceosome is a large ribonucleoprotein machine comprised of 5 small nuclear RNAs (snRNAs) and ~80 proteins. Assembly of the spliceosome requires a complex series of rearrangements fueled by members of the DEAD-box family of RNA-dependent ATPases. The spliceosome also contains a single essential GTPase, Snu114, whose function is poorly understood. Activation of the fully assembled spliceosome for catalysis requires the displacement of U1 and U4 snRNPs by the U5 snRNP-associated ATPases Prp28 and Brr2. An important unanswered question is how the activity of these ATPases is regulated. Based on recent genetic analyses, the Guthrie lab has proposed that Snu114 controls this regulation and, moreover, that the activity of Snu114 is itself regulated by ubiquitination. Specifically, their data suggest that GTP hydrolysis by Snu114 produces a large conformational rearrangement of the spliceosome via changed interactions between the C-terminal domain of Snu114 and the large U5 snRNP protein Prp8. Since Prp8 has previously been proposed to negatively regulate the activity of Prp28 and Brr2, this GTP-dependent rearrangement could likely allow activation of these ATPases. This proposal will test the hypotheses that Snu114-dependent GTP hydrolysis activates the spliceosome for catalysis and that ubiquitination is important for the function of Snu114. [unreadable] [unreadable] [unreadable]